Portable electric impact tool

ABSTRACT

A portable electric impact drill wherein an axially reciprocable tool spindle can be driven at two speeds by a gear transmission which is installed in front of a universal electric motor. The tool spindle is movable axially by a rotary or reciprocable selector to assume one of four positions including a first position in which it receives torque from a first gear of the transmission and is driven at a relatively low speed, a second position in which it receives torque from a second gear of the transmission and is driven at a relatively high speed, a third position in which it is disengaged from both gears, and a fourth position in which it receives torque from the second gear and causes an annulus of projections on the second gear to travel along an annulus of projections in the housing of the power tool so that the tool spindle rotates at the higher speed and is caused to perform axial oscillatory movements.

United States Patent [191 Armbruster Jan. 15, 1974 PORTABLE ELECTRIC IMPACT TOOL Prima Examiner-Marvin A. Cham ion [75] Inventor: Gerhard Armbruster, p

Asszstant Examiner-William F. Pate, lll Stuttgart Phemngen Germany Attorney-Michael S. Striker [73] Assignee: Robert Bosch Gmbll, Stuttgart,

Germany [22] Filed: Nov. 24, 1972 [21] Appl. No.: 308,941

[30] Foreign Application Priority Data Nov. 24, 1971 Germany P 21 58 118.0

[52] 11.5. C1 173/48, 74/342, 74/371, 173/97, 173/117 [51] Int. Cl B25d 15/00 [58] Field of Search 173/47-48, 94-97, 117, 163; 74/342, 371

[56] References Cited UNITED STATES PATENTS 3,396,593 8/1968 Moores, Jr 74/371 2,911,841 11/1959 Miller 74/342 3,680,642 8/1972 Kim et a1 173/48 X [57] ABSTRACT A portable electric impact drill wherein an axially reciprocable tool spindle can be driven at two speeds by a gear transmission which is installed in front of a universal electric motor. The tool spindle is movable axially by a rotary or reciprocable selector to assume one of four positions including a first position in which it receives torque from a first gear of the transmission and is driven at a relatively low speed, a second position in which it receives torque from a second gear of the transmission and is driven at a relatively high speed, a third position in which it is disengaged from both gears, and a fourth position in which it receives torque from the second gear and causes an annulus of projections on the second gear to travel along an annulus of projections in the housing of the power tool so that the tool spindle rotates at the higher speed and is'caused to perform axial oscillatory movements.

18 Claims, 6 Drawing Figures PATENTEDMN 1 5 mm SlliU 2 [If 1 PORTABLE ELECTRIC IMPACT TOOL BACKGROUND OF THE INVENTION The present invention relates to portable powertools in general, especially to electric power tools, and more particularly to imrprovements in portable electric impact drills, wrenches or the like. Still more particularly, the invention relates to improvements in electric impact tools wherein the tool spindle can be driven at a plurality of speeds.

A drawback of presently known portable electric power tools wherein the tool spindle can receive axial impulses and can be driven at several speeds is that the transmission for the tool spindle is bulky, expensive and consists of a large number of precision-finished parts. Furthermore, the speed changes must be effected by rotating, shifting and/or otherwise moving several discrete parts so that the manipulation of such power tools cannot be entrusted to unskilled operators.

SUMMARY OF THE INVENTION An object of the invention is to provide a portable power tool, especially an electric impact drill,which is simpler, less expensive, lighter, more compact and easier to manipulate than presently known power tools.

Another object of the invention is to provide a portable electric impact tool with a novel and improved multispeed transmission for the tool spindle.

A further object of the invention is to provide the electric impact tool with novel and improved means for selecting the speeds of the tool spindle and for moving the tool spindle to at least one further position in which the tool spindle is caused to perform axial oscillatory movements.

An additional object of the invention is to provide a portable electric impact tool with a multi-speed transmission for the tool spindle which can be manipulated not only by skilled operators but also by tinkers and other amateurs.

Still another object of the invention is to provide a portable electric impact drill or an analogous tool with novel and improved means for mounting the tool spindle, parts of the multi-speed transmission for the tool spindle and other components in the interior of the housing.

An ancillary object of the invention is to provide the power tool with a novel and improved tool spindle.

The improved portable power tool can be used with particular advantage as an impact drill and preferably embodies an electric motor. The power tool comprises a preferably plastic housing, a tool spindle which is rotatably and axially movably mounted in the housing and is connected with a chuck or analogous tool retaining means, a multi-speed transmission which is installed in the housing and includes first and second gears mounted on the tool spindle and means for rotating the first and second gears at different speeds, first coupling means provided on each of the gears, second coupling means provided on the tool spindle, axial impulse transmitting means provided in the housing, and selector means which is actuatable to move the tool spindle between a plurality of positions including a first position in which the coupling means of the tool spindle receives torque from the coupling means of the first gear, a second position in which the coupling means of the tool spindle receives torque from the coupling means of the second gear and a third position in which the tool spindle receives torque from one of the gears and receives axial impulses from the impulse transmitting means (preferably by way of the one gear) so as to perform simultaneous angular and axial oscillatory movements. If desired, the tool spindle can be moved to a fourth position in which its coupling means is disengaged from the coupling means of the first and second gears so that the gears can be driven by the motor while the tool spindle remains at a standstill.

The impulse transmitting means may comprise a first annulus of projections provided on a stationary portion of the housing and the one gear is preferably provided with a second annulus of projections which ride over the projections of the first annulus to thereby transmit to the tool spindle axial impulses while the coupling means of the tool spindle receives torque from the coupling means of the one gear.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved power tool itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal vertical sectional view of a portable electric impact drill which embodies one form of the invention, the section being taken in the direction of arrows as seen from the line 1-1 of FIG. 2;

FIG. 2 is a transverse vertical sectional view as seen in the direction of arrows from the line IIII of FIG.

FIG. 3 is a fragmentary longitudinal vertical sectional view of a second power tool;

FIG. 4 is a similar fragmentary longitudinal vertical sectional view of a third power tool;

FIG. 5 is a longitudinal vertical sectional view of the front portion of a fourth power tool; and

FIG. 6 is a fragmentary longitudinal vertical sectional view of the front portion of a fifth power tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 2, there is shown a portable electric power tool which constitutes an impact drill. The power tool comprises a housing including an outer portion consisting of two shells 1, 2 made of synthetic plastic material and defining a chamber which accommodates a universal electric motor 6 and a twospeed transmission which can driven an axially movable rotary tool spindle 17 at a higher speed and a lower speed. The rear portions of the shells 1 and 2 form a handle 3 having an opening for a cable 4 provided with a plug (not shown) which is connectable to an outlet. The motor 6 can be started in response to depression of a switch trigger 5.

The electric motor 6 comprises a rotor 7 which is secured to the rear portion 8 of an output shaft. The latter is formed with a blind axial bore for an insulating sleeve 9 which receives the rear part of the front shaft portion 10. The shaft portion 10 extends forwardly beyond the sleeve 9 and carries a fan 11 which serves to cool the interior of the housing. The rear shaft portion 8 has a smaller-diameter rear end 12 which is received in a sleeve 13 mounted in the housing and consisting of sintered metal. The front shaft portion extends forwardly beyond the fan 11 and is rotatable in a needle bearing 14 which is installed in a bridge member 15 secured to the shell 1 by two screws 16. The bridge member 15 is installed in the chamber which is defined by the outer portion 1, 2 of the housing and can be said to form part of this housing.

The tool spindle 17 is also mounted in the bridge member 15 and its rear end portion 17' is of semispherical outline. A first intermediate shaft 18 of the twospeed transmission is a press fit in a bore of the bridge member 15 and carries a gear cluster 21 consisting of sintered metal and including a smaller gear 20 and a larger gear 19. The gear cluster 21 is rotatable on the intermediate shaft 18in front of the bridge member 15. The larger-diameter gear 19 meshes with a small gear or pinion 22 constituting the foremost end of the output shaft portion 10 and being located in front of the needle bearing 14 in the bridge member 15. The pinion 22 further meshes with a gear 23 which is rotatable on the tool spindle 17 in front of the bridge member 15 and is located behind a similar gear 31. The latter is also rotatable on the tool spindle 17 and meshes with a gear 34 which is driven by the gear 20 of the gear cluster 21 and is rotatable on a second intermediate shaft 33 a portion of which is a press fit in a bore of the bridge member 15. The gear 31 is mirror symmetrical to the gear 23 with reference to a plane which is located between the gears 23, 31 and is normal to the axis of the tool spindle 17. At least one of the gears 23, 31 may consist of sintered metal. The spindle 17 is movable axially of the gears 23 and 31. The rear end face of the rear gear 23 on the tool spindle 17 is provided with an annulus of projections 24 which can travel along a similar annulus of axial impulse transmitting projections 25 provided on the front face of the bridge member 15. The front face of the rear gear 23 is further provided with a circular recess 23' the bottom wall of which is formed with a diametrically extending groove 26. The surface surrounding the groove 26 constitutes a torque transmitting coupling element of the gear 23 and can rotate the too] spindle 17 by way of a torque receiving coupling element 28 here shown as a pin received in a transverse bore 27 of the tool spindle 17 and extending with bothends beyond the peripheral surface of the tool spindle so as to be receivable in the groove 26 in two predetermined axial positions of the tool spindle. Analogously, the rear end face of the front gear 31 on the tool spindle 17 is provided with a circular recess 31 the bottom wall of which has a diametrically extending groove 32. The surface surrounding the groove 32 constitutes a torque transmitting coupling element of the gear 31 and can rotate the tool spindle 17 by way of the coupling element 28 in a further predetermined axial position of tool spindle. The combined axial length of the circular recesses 23, 31' exceeds the diameter of the coupling element 28 so that the tool spindle 17 can be moved to still another axial position in which the coupling element 28 is located in the recesses 23, 31 without extending into the groove 26 and/or 32. In such axial position, the spindle 17 is at a standstill even if the gears 23, 31 rotate. The axis of the transverse bore 27 preferably intersects the axis of the tool spindle 17, and the length of both outwardly projecting end portions of the coupling element 28 is preferably the same.

The tool spindle 17 extends forwardly beyond the gears 23, 3'1 and is rotatable and axially movable in a bearing sleeve 30 which is mounted in a neck portion of the housing including the shells 1, 2 and preferably consists of sintered metal. The foremost part of the tool spindle 17 is rigidly connected to a conventional chuck 29 which can take and rotate a screwdriver, a boring tool, a wrench or any other tool capable of performing work in response to rotation and/or axial oscillatory movements of the tool spindle.

The axial length of that portion of the intermediate shaft 18 which extends forwardly beyond the bridge member 15 preferably equals the axial length of the corresponding portion of the intermediate shaft 33. The front portions of the intermediate shafts 18 and 33 have reduced diameters (see the portion 18' of the intermediate shaft 18 in FIG. 1) and are received in apertures provided in a metallic plate-like holder 35. Those parts of the intermediate shafts l8 and 33 which extend beyond the respective apertures of the holder 35 are upset to respectively form rivets 18" and 33". The purpose of the holder 35 is to hold the gears 20, 34 and 31 against axial movement and to prevent sidewise movements of intermediate shafts 18, 33 relative to each other.

The rear face of the bridge member 15 is provided with a blind bore for the front end portion of a shaft 36 which is a press fit therein and the rear end portion of which extends into a blind bore 37 of the shell 1. The intermediate portion of the shaft 37 supports rotary selector knob 38 having a serrated, knurled or otherwise roughened peripheral surface 39 a portion of which extends outwardly through a window 40 of the shell 1 so that it can be reached by a finger of the operator. The rear end face of the knob 38 is formed with a set of four detent notches 41 (only one shown in FIG. 1) each of which can be moved into register with and then receives a portion of a detent ball 44 biased toward the knob 38 by a helical spring 43 and being received in a blind bore 42 of the shell 1. The knob 38 is provided with four notches 41 (all of which are equidistant from the axis of the shaft 37) because the tool spindle 17 is assumed to be movable between four different axial positions. The rear face of the knob 38 preferably abuts against that portion of the surface of the shell 1 which bounds the rearmost part of the window 40. The ball 44 preferably consists of steel and is mounted in the bore 42 of the shell 1 in such a way that it automatically enters into and is expelled from successive notches 41 in response to rotation of the knob 38.

A portion of the selector knob 38 constitutes a cam having an arcuate cam face 38 which is formed with four steps and can be tracked by the semispherical rear end portion or follower 17 of the tool spindle 17. Thus, by the simple expedient of rotating the knob 38, the operator of the power tool can effect an axial movement of the tool spindle 17 to or from any one of four different axial positions. The four steps of the cam face 38 are preferably connected to each other by inclined guide surfaces (not specifically shown) along which the follower 17 slides in response to rotation of the knob 38. When the power tool is in use, the top of the tool which is mounted in the chuck 29 normally bears against a workpiece so that the tool spindle 17 (which is rigid with the housing of the chuck 29) tends to move rear-wardly (toward the universal motor 6) and thus maintains the follower 17' in abutment with the cam face 38.

The selector knob 38 can move the tool spindle 17 to any one of the following four axial positions:

In a first axial position, the tool spindle 17 is remotest from the motor 6 and maintains its coupling element 28 in the groove 32 of the front gear 31. The tool spindle 17 is then rotated at a relatively slow speed by way of the front output shaft portion 10, pinion 22, gear cluster 21, gear 34 and gear 31.

In a second axial position of the tool spindle 17, the coupling element 28 is located between the grooves 26, 32 (i.e., in the recesses 23, 31') and is at a standstill even if the gears 23 and 31 are driven by the motor 6.

In a third axial position, the tool spindle 17 maintains its coupling element 28 in the groove 26 of the gear 23 so that it is driven at a relatively high speed by way of the shaft portion 10, pinion 22, gear cluster 21 and gear 23.

In the fourth axial position, the tool spindle maintains its coupling element 28 in the groove 26 of the gear 23 so that it is driven at a relatively high speed. At the same time, the tool spindle l7 urges the gear 23 rearwardly (by way of the coupling element 28) so that the projections 24 of the gear 23 ride over the stationary projections 25 of the bridge member and causes the gear 23 to oscillate with the tool spindle 17 by moving therewith axially back and forth while the coupling element 28 remains in the groove 26 so that the gear 23 continues to drive the tool spindle. The thus received impulses are communicated to the tool which is mounted in the chuck 29 so that the tool is more likely to penetrate into the material of a workpiece. The configuration of the cam face 38' on the selector knob 38 is such that, when the tool spindle 17 assumes its forth axial position, it has sufficient freedom of axial oscillatory movement while the projections 24 of the gear 23 travel along the projections 25 of the bridge member 15.

The gear 34 may consist of sintered metal.

For the parts consisting of sintered metal, sintered pulverized unalloyed steel presently is preferred.

FIG. 3 illustrates a modified construction of the rear end portion ofa tool spindle 117. This rear end portion has a socket 117A for a spherical follower 17a which tracks the cam face 38 of the rotary selector knob 38 on the shaft 36. The spherical follower 17a preferably consists of hardened steel or the like. The socket 117A is initially formed as an axial blind bore to receive a little more than one-half of the spherical follower 17a; in the next step, the marginal portion of the spindle 117 at the open end of the blind bore is upset to prevent uncontrolled escape of the follower 17a from the thus obtained socket 117A.

FIG. 4 shows the rear end portion of a third tool spindle 217 which is formed with an axial blind bore 17b the innermost part of which receives a steel ball 17d. This ball constitutes an anvil for the rounded front end portion of an elongated cylindrical pin 17c which consists of hardened steel or the like and has a semispherical rear end portion constituting a follower which tracks the cam face 38' of the selector knob 38. The pin 17c is reciprocable in the axial bore 17b of the tool spindle 217.

Flg. 5 illustrates the front portion of a modified power tool wherein the tool spindle 317 is formed with a circumferential groove l7e located in front of the gear 31 and receiving a split ring 17f serving as a re tainer for the smallest rearmost convolution of a conical helical spring 17g bearing against an internal surface of the outer housing portion formed by the shell 1 and/or 2. If desired, a thrust bearing can be interposed between the rearmost convolution of the spring 17g and the split ring 17f. The purpose of the spring 17g is to bias the tool spindle 317 against the cam face 38 of the knob 38 even if the tool which is mounted in the chuck (not shown) does not engage a workpiece and even if the power tool is held in such position that the chuck is located at its lower end, i.e.. that the tool spindle 317 tends to move away from the cam face 38 under the action of gravity.

FIG. 5 shows the tool spindle 317 in the aforediscussed third axial position in which the coupling element 28 receives torque from the coupling element of the rear gear 23 so that the spindle 317 is driven at the higher of two speeds as soon as the operator depresses the switch trigger 5 (FIG. 1). The spring 17;; is preferably disengaged from the split ring 17f when the tool spindle 317 is moved to the fourth axial position in which it receives impulses from the projections 25 of the bridge member 15.

In FIG. 6, the helical spring 17g has been replaced by a leaf spring 17h which bears against the split ring 17f and tends to maintain the tool spindle 317 in the third axial position. The bias of the leaf spring 17h is prefera bly such that the spindle 317 cannot move forwardly from its second or third axial position except, of course, in response to rotation of the selector knob 38 (not shown in FIG. 6).

The rotary selector knob 38 may be replaced by a reciprocable selector element which can move lengthwise between several positions each of which corresponds to a different axial position of the tool spindle. Such reciprocable selector element is preferably movable in parallelism with the axis of the tool spindle.

An important advantage of the improved power tool is that it consists of a relatively small number of simple, rugged and long-lasting parts in spite of the fact that the housing receives a multi-speed transmission. Another advantage of the improved power tool is that the axial' positions of the tool spindle can be changed by means of a single selector in a time-saving operation and with the exertion of a minimal effort.

An advantage of the power tool which does not employ the biasing means 17g or 17h is that the operator can cause the tool spindle to automatically assume its first or second axial position by the simple expedient of tilting the power tool forwardly so that the tool spindle can slide under the action of gravity. Thus, and assuming that the motor 6 is on and that the tool spindle 17 of FIG. 1 dwells in its second axial position, a simple forward tilting of the power tool will cause the coupling element 28 to find its way into the recess 32 and to thus receive torque from the gear 31 whereby the spindle 17 rotates at the lower of two speeds. The selector knob 38 can be rotated thereafter to retain the tool spindle 17 in the first axial position. If the knob 38 is not rotated after the tool spindle 17 has assumed its first axial position, the coupling element 28 leaves the groove 32 as soon as the tip of a tool in the chuck 29 is pressed against a workpiece so that the spindle assumes an axial position which is dictated by the selected setting of the knob 38.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly consitute essential characteristics of the generic and specific aspects of my contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

I claim:

1. In a portable power tool, particularly in a portable electric impact drill, a combination comprising a housing; a tool spindle rotatably and axially movably mounted in said housing; a multi-speed transmission provided in said housing and including first and second gears rotatably mounted on said spindle and means for rotating said gears at different speeds, each of said gears having torque transmitting first coupling means and said spindle having torque receiving second coupling means; axial impulse transmitting means provided in said housing; and selector means actuatable to effect axial movements of said spindle between a plurality of positions including first and second positions in which said second coupling means respectively engages with and receives torque from the first coupling means of said first and second gears and a third position in which said second coupling means engages the first coupling means of one of said gears and at least said spindle receives axial impulses from said impulse transmitting means so that said spindle oscillates axially while being driven by said one gear.

2. A combination as defined in claim 1, wherein said one gear is movable with said tool spindle while the spindle moves axially from the respective one of said first and second axial positions to said third position, said impulse transmitting means being arranged to oscillate said spindle by way of said one gear in said third position of said spindle.

3. A combination as defined in claim 2, wherein said impulse transmitting means comprises a first annulus of projections provided in said housing and said one gear comprises a second annulus of projections which travel along the projections of said first annulus in said third position of said spindle.

4. A combination as defined in claim 1, further comprising electric motor means mounted in said housing and having a rotary output element arranged to drive said means for rotating said gears at different speeds.

5. A combination as defined in claim 1, further comprising means for biasing said tool spindle to at least one of said positions thereof.

6. A combination as defined in claim 1, wherein said tool spindle is provided with follower means at one end thereof and said selector means comprises a rotary element mounted in said housing and having a cam face tracked by said follower means, said rotary element being turnable by hand to thereby change the axial position of said spindle by way of said cam face and said follower means.

7. A combination as defined in claim 6, wherein said housing is provided with an opening and said rotary element has a roughened peripheral surface a portion of which extends outwardly through said opening.

8. A combination as defined in claim 6, wherein the configuration of said cam face is such that said follower means has freedom of limited axial movement relative to said rotary element in said third position of said spindle so that said spindle is free to oscillate while receiving torque from said one gear.

9. A combination as defined in claim 1, wherein said selector means is slidable relative to said housing between aplurality of positions each of which corresponds to a different axial position of said tool spindle.

10. A combination as defined in claim 1, wherein said housing includes an outer portion defining a chamber and a bridge member provided in said chamber, said spindle having a portion rotatable and axially movable in said bridge member and said impulse transmitting means being provided on said bridge member.

11. A combination as defined in claim 10, further comprising electric motor means installed in said chamber and having a rotary output element arranged to drive said means for rotating said gears at different speeds and being rotatably journaled in said bridge member.

12. A combination as defined in claim 11, wherein said means for rotating said gears at different speeds comprises an intermediate shaft mounted in said bridge member and a third gear mounted on said intermediate shaft and meshing with said first gear.

13. A combination as defined in claim 12, wherein said means for rotating said first and second gear at different speeds further comprises a second intermediate shaft mounted in said bridge member and a fourth gear mounted on said second intermediate shaft and meshing with said second gear.

14. A combination as defined in claim 13, wherein said selector means comprises a rotary element and a shaft for said rotary element, said shaft being mounted in said bridge member.

15. A combination as defined in claim 1, wherein said first and second gears are mirror symmetrical to each other with reference to a plane passing between said first and second gears and normal to the axis of said tool spindle.

16. A combination as defined in claim 1, wherein said housing comprises an outer portion defining a chamber for said transmission and a bridge member provided in said chamber and consisting of sintered metal, said spindle being rotatably and axially movably mounted in said bridge member.

17. A combination as defined in claim 1, wherein at least one of said first and second gears consists of sintered metal.

18. A combination as defined in claim 1, wherein said means for rotating said first and second gears comprises additional gear means consisting of sintered metal. 

1. In a portable power tool, particularly in a portable electric impact drill, a combination comprising a housing; a tool spindle rotatably and axially movably mounted in said housing; a multispeed transmission provided in said housing and including first and second gears rotatably mounted on said spindle and means for rotating said gears at different speeds, each of said gears having torque transmitting first coupling means and said spindle having torque receiving second coupling means; axial impulse transmitting means provided in said housing; and selector means actuatable to effect axial movements of said spindle between a plurality of positions including first and second positions in which said second coupling means respectively engages with and receives torque from the first coupling means of said first and second gears and a third position in which said second coupling means engages the first coupling means of one of said gears and at least said spindle receives axial impulses from said impulse transmitting means so that said spindle oscillates axially while being driven by said one gear.
 2. A combination as defined in claim 1, wherein said one gear is movable with said tool spindle while the spindle moves axially from the respective one of said first and second axial positions to said third position, said impulse transmitting means being arranged to oscillate said spindle by way of said one gear in said third position of said spindle.
 3. A combination as defined in claim 2, wherein said impulse transmitting means comprises a first annulus of projections provided in said housing and said one gear comprises a second annulus of projections which travel along the projections of said first annulus in said third position of said spindle.
 4. A combination as defined in claim 1, further comprising electric motor means mounted in said housing and having a rotary output element arranged to drive said means for rotating said gears at different speeds.
 5. A combination as defined in claim 1, further comprising means for biasing said tool spindle to at least one of said positions thereof.
 6. A combination as defined in claim 1, wherein said tool spindle is provided with follower means at one end thereof and said selector means comprises a rotary element mounted in said housing and having a cam face tracked by said follower means, said rotary element being turnable by hand to thereby change the axial position of said spindle by way of said cam face and said follower means.
 7. A combination as defined in claim 6, wherein said housing is provided with an opening and said rotary element has a roughened peripheral surface a portion of which extends outwardly through said opening.
 8. A combination as defined in claim 6, wherein the configuration of said cam face is such that said follower means has freedom of limited axial movement relative to said rotary element in said third position of said spindle so that said spindle is free to oscillate while receiving torque from said one gear.
 9. A combination as defined in claim 1, wherein said selector means is slidable relative to said housing between a plurality of positions each of which corresponds to a different axial position of said tool spindle.
 10. A combination as defined in claim 1, wherein said housing includes an outer portion defining a chamber and a bridge member provided in said chamber, said spindle having a portion rotatable and axially movable in said bridge member and said impulse transmitting means being provided on said bridge member.
 11. A combination as defined in claim 10, further comprising electric motor means installed in said chamber and having a rotary output element arranged to drive said means for rotating said gears at different speeds and being rotatably journaled in said bridge member.
 12. A combination as defined in claim 11, wherein said means for rotating said gears at different speeds comprises an intermediate shaft mounted in said bridge member and a third gear mounted on said intermediate shaft and meshing with said first gear.
 13. A combination as defined in claim 12, wherein said means for rotating said first and second gear at different speeds further comprises a second intermediate shaft mounted in said bridge member and a fourth gear mounted on said second intermediate shaft and meshing with said second gear.
 14. A combination as defined in claim 13, wherein said selector means comprises a rotary element and a shaft for said rotary element, said shaft being mounted in said bridge member.
 15. A combination as defined in claim 1, wherein said first and second gears are mirror symmetrical to each other with reference to a plane passing between said first and second gears and normal to the axis of said tool spindle.
 16. A combination as defined in claim 1, wherein said housing comprises an outer portion defining a chamber for said transmission and a bridge member provided in said chamber and consisting of sintered metal, said spindle being rotatably and axially movably mounted in said bridge member.
 17. A combination as defined in claim 1, wherein at least one of said first and second gears consists of sintered metal.
 18. A combination as defined in claim 1, wherein said means for rotating said first and second gears comprises additional gear means consisting of sintered metal. 